The invention relates to a process for production of diaryl carbonate combined with the electrolysis of the resultant alkali metal chloride-containing process wastewater. The process according to the invention makes possible, inter alia, improved utilization in electrolysis of the alkali metal chloride-containing solution obtained in the production of diaryl carbonate.
The production of diaryl carbonates (diaryl carbonate) proceeds conventionally by a continuous process, by production of phosgene and subsequent reaction of monophenols and phosgene in an inert solvent in the presence of alkali and a basic nitrogen catalyst in the interface.

The production of diaryl carbonates, for example by the phase boundary process, is described in principle in the literature, see, for example, in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pp. 50/51.
U.S. Pat. No. 4,016,190 describes a production process for diaryl carbonates which is operated at temperatures >65° C. The pH in this process is first set to be low (pH 8 to 9) and subsequently high (10 to 11).
Optimization of the process by improving the mixing, and maintaining a narrow temperature and pH profile, and also isolation of the product, are described in EP 1 219 589 A1, EP 1 216 981 A2, EP 1 216 982 A2 and EP 784 048 A1.
However, in these known processes, a high residual phenol value in the wastewater of these processes, which can pollute the environment and give the effluent treatment plants increased wastewater problems, makes complex purification operations necessary. For instance, WO 03/070639 A1 describes removal of the organic impurities in wastewater by an extraction with methylene chloride.
Conventionally, alkali metal chloride-containing, preferably sodium-chloride-containing, solution is freed from solvents and organic residues and must then be disposed of.
However, it is also known that purification of the sodium-chloride-containing waste-waters can proceed according to EP 1 200 359 A1 or U.S. Pat. No. 6,340,736 via ozonolysis and is then suitable for use in sodium chloride electrolysis. A disadvantage of ozonolysis is that this process is very cost intensive.
According to EP 541 114 A2, a sodium-chloride-containing wastewater stream is evaporated up to complete removal of the water and the remaining salt having the organic impurities is subject to a thermal treatment, as a result of which the organic components are destroyed. Particular preference here is given to the use of infrared radiation. A disadvantage of the process is that the water must be completely evaporated so that the process cannot be carried out economically.
According to WO 03/70639 A1, the wastewater from a DPC production is purified by extraction and then fed to the sodium chloride electrolysis. However, by means of the process described, only a maximum of 26% of the sodium chloride can be recovered from the wastewater of the DPC production, since, at higher feed rates, the water introduced into the electrolysis together with the wastewater would bring the water balance of the sodium chloride electrolysis out of equilibrium.
The alkali-metal-chloride-containing, preferably sodium-chloride-containing, solutions which are obtained in DPC production, typically have an alkali metal chloride content, preferably sodium chloride content, of 13 to 17% by weight. It is not possible thereby to recover all of the alkali metal chloride present in the solutions in this manner. At an alkali metal chloride concentration of 17% by weight, in the standard alkali metal chloride electrolysis, preferably standard sodium chloride electrolysis, using a commercially available ion-exchange membrane which exhibits a water transport of 3.5 mol of water per mole of sodium, only the use of approximately 23% of the sodium chloride from the sodium-chloride-containing solutions succeeds. Even with concentration up to a saturated sodium chloride solution of approximately 25% by weight only give a recycling rate of 38% of the sodium chloride contained in the sodium-chloride-containing solution. Complete recycling of the alkali-metal-chloride-containing solution is not known to date. According to WO 01/38419 A1, the sodium-chloride-containing solution can be evaporated by means of thermal processes, so that a highly concentrated sodium chloride solution can be fed to the electrolysis cell. However, the evaporation is energy intensive and expensive.
Proceeding from the above-described prior art, the object of the invention was to provide a process for production of diaryl carbonate which delivers products in high purity and good yield and at the same time makes possible reduction of the environmental pollution and/or wastewater problems in the sewage treatment plants by maximal recycling of alkali metal chloride from alkali-metal-chloride-containing process wastewater solutions which are obtained from the diaryl carbonate production.
In particular, in the recycling, it should be taken into account that the reaction of alkali metal chloride, preferably sodium chloride, to give chlorine and alkali solution, preferably sodium hydroxide solution, and optionally hydrogen, should proceed with minimum energy use and therefore likewise be sparing of resources.
It has surprisingly been found that in the production of diaryl carbonates by reaction of monophenols and phosgene in an inert solvent in the presence of a base and optionally a basic catalyst, improved utilization of the alkali-metal-chloride-containing solution resulting from the production of the diaryl carbonate can be achieved in a chlor-alkali electrolysis, when the alkali-metal-chloride-containing solution resulting from the production of diaryl carbonates has an alkali metal chloride content of 18 to 25% by weight, based on the total weight of the alkali-metal-chloride-containing solution. Such an alkali metal chloride content of 18 to 25% by weight of the alkali-metal-chloride-containing solution which is obtained in the production of diaryl carbonates can be achieved according to the invention firstly by the means that, in the reaction of monophenol and phosgene in the presence of a base, the monophenol and an alkali-metal-containing base are used in amounts such that the sodium phenolate content of the resultant solution of alkali-metal-containing base and monophenol is in a specially selected range, preferably 31 to 40% by weight sodium phenolate, based on the total weight of the solution. In addition, or alternatively, at least a part of the alkali-metal-chloride-containing solution which is obtained in the production of diaryl carbonates can be recycled to the production of diaryl carbonates, for example by replacing any water to be used in order likewise to achieve concentration of the alkali-metal-chloride-containing solution obtained in the production of diaryl carbonates to an alkali metal chloride content of 18 to 25% by weight.
This is surprising, since an elevation of the sodium phenolate content in the starting solution in the production of diaryl carbonates in the subsequent exothermal formation of diaryl carbonates should lead to an increased occurrence of energy and consequently to increased by-product formation. In addition, in the recycling of the sodium-chloride-containing solution to the diaryl carbonate production, for example by replacing any water to be used, owing to the presence of alkali metal chloride in the starting material solution, a decrease in the solubility of phenol and corresponding precipitation must be expected. Surprisingly, neither were observed within the range selected according to the invention of an alkali metal chloride content of 18 to 25% by weight, based on the total weight of the alkali-metal-chloride-containing solution. In contrast, the process according to the invention surprisingly offers the possibility of recycling by means of electrolysis significantly greater fractions of the alkali metal chloride situated in the reaction wastewater of the diaryl carbonate production, and thereby of utilizing it.